1. Field of the Invention
The present invention relates to a novel dichroic dye compound, composition thereof, and the liquid crystal composition and the liquid crystal display element containing the compound of the invention. In particular, the dichroic dye compound of the invention can be used in photoelectric products, for example, liquid crystal displays, especially reflective liquid crystal displays.
2. Description of the Related Art
There is an increasing need for reflective liquid crystal displays for use in information products, such as E-books and PDAs, because they consume low electric energy and are thin and light. Currently, the guest-host display mode is applied in reflective liquid crystal displays, dye being “guest” and liquid crystal being “host”. The liquid crystal display effects are generated by the anisotropically optical absorption by dichroic dye which is dissolved in the mixture of nematic liquid crystal and an optically active substance. This display mode employs absorption or no absorption for visible light by dye, exhibits bright and dark corresponding to the variation of electric field and may replace reflectors. The view angle of this type of reflective liquid crystal display is improved and the brightness is also increased. When used, only environmental light is required, but not back light, therefore electric consumption can be greatly reduced.
In guest-host display mode, dichroic dyes are used and must exhibit good dichroism, which can be quantified by the orientational order parameter (SD) or dichroic ratio (N) of the dye molecules. The contrast is higher when the values of N and SD are higher. Moreover, the dye molecules in the display devices tend to deteriorate due to long term exposure to irradiation of back light or environmental light (such as sun light), and the display quality is affected adversely. Accordingly, it is desirable for the dyes to have good photostability and heat stability for enhanced durability. Furthermore, the solubility of dichroic dye in liquid crystal is generally 1%˜10% based on the weight of liquid crystal. The dichroic dye rotates as the liquid crystal rotates in accordance with the increase of applied electric voltage. To increase the contrast of the display, an increase in solubility and load of dichroic dye in liquid crystal is desirable to fulfill the prerequisite of avoiding reduced brightness and increased threshold electric voltage.
However, few conventional dichroic dyes meet all the requirements mentioned above. Conventional dichroic dyes, those which meet the basic requirements for dichroism and coloration, often have poor processability because of poor solubility or short product life because of poor photostability.
For example, the photostability and heat stability of general azo dyes are poor, but those of anthraquinone dyes are relatively stable due to the hydrogen bond between molecules. The mono-azo dye having heterocyclic julolidine or thiazole group has relatively high photostability, but reduced solubility. U.S. Pat. No. 4,304,683 discloses an anthraquinone dye having a long linear or branched soft hydrocarbyl group, which has an asymmetric molecular structure, but the solubility is poor. In EP 0098522, the soft hydrocarbyl group is replaced by dialkylamino group and this can improve solubility of the dichroic dyes. Liquid Crystals, 2002, 29, pp707-712 reports the solubility of ester-disazo dyes can be improved by the introduction of multi-fluorine atoms.
Therefore, there is still a need for dichroic dyes with high solubility and stability.